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Betelgeuse - Alpha Orionis - α Ori

Betelgeuse is a red supergiant in Orion that's one of the most famous stars in the sky. It's an irregular variable that usually fluctuates between magnitudes +0.3 and +0.8, though on rare occasions it has been known to peak at magnitude 0.0 and dim down to magnitude +1.2. The star currently hovers around magnitude +0.42. It marks the upper left-hand corner of the ancient hunter figure.

Although lettered alpha (α) Orionis, Betelgeuse is usually fainter than Rigel (β Orionis - mag. +0.13) and is therefore the constellations second brightest star. Sir John Herschel, in 1836, was the first person to record brightness variations and on two occasions, in October 1837 and November 1839, he observed it to be brighter than Rigel. At best, Betelgeuse is comparable in brightness to Rigel and Capella (α Aur - mag. +0.08) but when towards the lower end of its range, appears more like Aldebaran (α Tau - mag(v). +0.75 -> +0.85).

The name Betelgeuse is Arabic and is derived from "Ibt-al-Jauza", which means "the shoulder of the central one". It's alternatively written as Betelgeuze or Betelgeux and pronounced in a number of ways; many people refer to it as "Beetlejuice", though "Beteljerze" is closer to the original Arabic pronunciation. Eminent English 19th century astronomer William Lassell - who discovered Neptune's largest moon Triton, Saturn's moon Hyperion (co-discovered) and Uranus moon's Ariel and Umbriel - majestically described Betelgeuse as "a most beautiful and brilliant gem; a rich topaz in hue and brilliancy different from any other star I have ever seen".

As with all M type stars the surface temperature of Betelgeuse is much cooler than the Sun, about 3,400 K compared to 5,800 K. Betelgeuse is truly enormous in size, with a diameter that could be as large as 1.67 billion kilometres (1.04 billion miles) or about 1,200x that of the Sun. If placed at the centre of the Solar System, it would easily contain the orbits of Mercury, Venus, Earth and Mars. However, since Betelgeuse is much less dense that the Sun, it's only between 8 and 20 times more massive. It's also likely that the star has used up its supply of hydrogen and is now fusing helium at its core. Eventually, it will explode as a supernova and this will be a spectacular event. The explosion will temporarily shine as bright as the crescent Moon, cast strong shadows on the ground and be easily visible in daylight. It could go bang tomorrow, but we may have to wait another million years or so. Only time will tell.

Betelgeuse is located 640 light-years distant and is the nearest of the vast red supergiants. As a result, it was the first star to be directly imaged by the Hubble Space Telescope (in 1995). It has an apparent diameter of 0.05 arc seconds. The star is about 10,000x more luminous than the Sun.

The chart below shows its position in Orion. Since located just north of the celestial equator, Betelgeuse can be seen from every permanently inhabited location worldwide. It's best seen during the months of December, January and February.